Not to confuse you further in your transition from classical to traditional, but this isn’t true. I could point you to dozens if not hundreds of recordings of traditional whistle players who articulate using tonguing (or glottal stops) and not exclusively with the fingers.
Right, but when you listen to uilleann pipers you will see that they have the ability to stop the chanter between notes, an effect that on the whistle can only be imitated by tonguing or glottal stopping – you can’t stop the sound with your fingers on a whistle the way you can on an uilleann pipes chanter.
It is possible to play the whistle using exclusively finger articulation, and some great whistle players do/did it, but many good players use finger articulation in combination with articulation achieved by tonguing and/or glottal stopping. Have a listen to Willie Clancy here:
http://www.rogermillington.com/tunetoc/maidonthegreen.html
Tell me that was all done with finger articulation! There is plenty of finger articulation, but quite a bit of tonguing (or possibly glottal stops, it’s not always easy to tell the difference) to separate notes as well.